The present invention relates to an apparatus for filling, pasteurizing, sterilizing, retorting, and sealing food product in a continuous manner, and in commercial quantities, in a food container which is pellucid to microwave radiant energy.
There is an increasing need for mass production of retort prepared food, which is suitable for consumer convenience and health, as for canned food. A number of well developed methods for retorting, sterilizing, filling and sealing such food are available. However, cans represent a relatively costly means for packaging, and hence, alternate packages are of great interest.
The temperature required for retorting and sterilization of food product is substantially above the boiling point of water at atmospheric pressure. Usually a temperature of about 120.degree. C. is preferred. At that temperature, the thermal death time of the bacteria is reduced to a matter of seconds. Food engineering teaches that the normal rate of destruction of bacteria usually increases 10 to 20 times, for each 10.degree. C. rise of temperature above boiling point. Furthermore, it is well known that some foodstuff maintain better quality when sterilized in a shorter time at higher temperature.
In high temperature-short time sterilization, a high rate of heat penetration in the food is needed. The retorting and sterilization of food using microwave energy is particularly suitable to obtain such high heat absorbtion. Therefore, since the commercialization of microwave ovens, considerable resources have been directed to the research and development of microwave apparatus capable of processing pre-packed dishes in large volumes, and in a continuous mode.
New discoveries have produced containers and vessels permeable to microwave radiant energy, with such preservation properties that they can replace tins as containers for foods. One example of such microwavable package is disclosed in Kinigakis et al U.S. Pat. No. 5,039,001, Aug. 13, 1991. Accordingly and concurrently, the demand for preparing foodstuff in non-metallic packages using microwave energy has increased considerably.
In a microwave environment, the food molecules become highly agitated and produce a "friction" which manifests itself by the immediate release of heat energy within the food and surrounding media. Upon the release of heat, pressure inside the container inevitably increases. The container could thereby be damaged if no protection is provided to counterbalance such internal pressure. Several prior art equipment use pressurized cooking chambers to prevent such container expansion.
Kenyon et al, U.S. Pat. No. 3,961,569, Jun. 8, 1976, discloses a continuous microwave sterilization apparatus which has a pressurized enclosure to prevent bursting of flexible pouches containing food.
Ruozi U.S. Pat. No. 5,074,200, Dec. 24, 1991, discloses another type of pressurized apparatus having different compartments and means of maintaining different pressure within different compartment,
Pressurized tunnels and tanks are further disclosed in Lipoma U.S. Pat. No. 3,718,082, Feb. 27, 1973 and in Wijts U.S. Pat. No. 4,773,321, Sep. 27, 1988.
In a closed container, the pressure is known to increase proportionally to the temperature rise within that container. Therefore, continuous microwave apparatus having uniformly pressurized chambers are unable of maintaining ideal pressure requirement during all phases of the process. For example, a pressure setting to countervail maximum internal forces in a container near the end of its cooking cycle, could collapse a container entering a microwave chamber where the internal temperature is almost ambient. The same damage will also occur when that container travels through a cooling chamber, and its temperature decreases substantially. Consequently, the pressure inside those pressurized enclosures is adjusted midway between bursting and collapsing pressures, hoping that the container will resist any residual forces.
Pressurized chambers suffers also from the inconvenience of complicated pressure holding doors, which are not particularly adapted for continuous processes.
Furthermore, continuous microwave apparatus having pressurized chambers are limited to the processing of sealed packages, containing all ingredients of a recipe. For example, sensitive meat like fish receives the same amount of energy as the surrounding sauce and vegetable within that container. The texture of the meat may suffer from over-cooking in order to obtain ideal exposure for carrots or broccoli. Therefore, these microwave apparatus are not practical to process multiple elements dishes.